Abstract : The Li isotope signatures of hydrothermal fluids are remarkably constant (View the MathML sourceδLi7=8.0±1.9‰) irrespective of the water/rock ratio (W/RW/R), permeability, temperature or fluid involved (seawater or meteoric). High temperature hydrothermal fluids represent the second most significant source of Li to the ocean, yet the homogeneity of the Li isotopic signatures of this source remains to be explained and in this context, the lack of data for the corresponding altered phases is problematic. We measured Li contents and Li isotope signatures (as well as mineralogy, composition and local fluid temperature) in hyaloclastites collected from a borehole in the Hellisheidi geothermal system (Iceland) which have been altered by high temperature aqueous fluids (from 170 to 300 °C). Li is more enriched in the solid phases than the other alkali metals, highlighting its greater ability to be incorporated into secondary phases, especially at high temperatures (>250 °C). Mass balance calculations show that the low Li concentrations in hydrothermal fluids are best explained by a high water/rock ratio and a high permeability of this system. The Li isotopic signature of the altered hyaloclastites (View the MathML sourceδLi7 between +1.9 and +4.0‰+4.0‰) remains close to the fresh basalt at deep levels and high temperatures (290–300 °C) (as measured View the MathML sourceδLi7 range between +3.7 and +4.0‰+4.0‰), and decreases at shallower depths and lower temperatures (150–270 °C) (View the MathML sourceδLi7 between +1.9 and +3.1‰+3.1‰). A mass balance model involving basalt dissolution, secondary phase formation, and successive isotope equilibrium during the migration and the cooling of the percolating fluid was developed. The corresponding apparent mineral-fluid Li isotope fractionation factors resulting from precipitation of secondary phases (View the MathML sourceΔLiminerals-fluid7) range between 0‰ at 300 °C and −8.5‰−8.5‰ at 170 °C and highlight a key role of chlorite. Applying the same approach to mid-ridge oceanic hydrothermal systems allows the relatively homogeneous isotope signatures of high temperature fluids of various locations to be explained.